Cloud condensation nuclei

From Glossary of Meteorology

cloud condensation nuclei

(Abbreviated CCN.) Hygroscopic aerosol particles that can serve as nuclei of atmospheric cloud droplets, that is, particles on which water condenses (activates) at supersaturations typical of atmospheric cloud formation (fraction of one to a few percent, depending on cloud type).

Concentrations of CCN need to be given in terms of a supersaturation spectrum covering the range of interest or at a specified supersaturation value.
Compare condensation nucleus.